Security Document IR Signature via Substrate Absorber
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Solution Overview
Problem
Existing security elements in valuable documents, such as banknotes, are vulnerable to 'harvesting' attacks where the security element is removed and transferred to counterfeit documents, making it difficult for machines to detect manipulation, especially since film-based security elements lack machine-readable features and IR absorbers often overlap in spectral ranges used by inexpensive sensors.
Innovation Solution
A valuable document with a substrate and a security element featuring an interference layer stack for an optically variable appearance and a subsurface IR absorber layer, which provides a characteristic IR signature and high adhesion to the substrate, ensuring that when the security element is detached, the absorber remains with the document, allowing for reliable machine detection of manipulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an IR absorber is printed on non-metallized areas of the foil element to enable machine detection, then the absence of the genuine security element can be detected with IR remission sensors, but this requires larger demetallized areas which represents a significant limitation of the foil design
Solution Approach 1:
The invention transitions from detecting security elements by their presence on the document surface to detecting them by their subsurface IR signature. By placing the IR absorber in the substrate beneath the security element rather than on the foil surface, the detection occurs through transmission through the security element, eliminating the need for large demetallized areas and resolving the foil design limitation.
2Reliability
If two different IR absorbers absorbing in spectrally separate ranges are used to mark the substrate and foil, then a characteristic combination signal can be provided for authenticity verification, but most IR absorbers absorb broadband and therefore spectrally overlap in the limited spectral range of commonly used inexpensive IR sensors
Solution Approach 1:
The invention extracts the IR absorber from the security element itself and places it solely in the substrate beneath the security element. This eliminates the need for two separate IR absorbers with spectrally separate absorption ranges, as only one IR absorber in the substrate is needed. The security element's presence is verified by detecting its characteristic effect on the IR signature from the single subsurface absorber, simplifying the spectral requirements for inexpensive sensors.
3Reliability
If the adhesion of the background layer containing the IR absorber to the substrate is made higher than the adhesion to the security element, then the second authenticity feature remains essentially completely on the value document substrate when the security element is removed, but this requires optimized adhesion properties
Solution Approach 1:
The invention applies different adhesion characteristics to different interfaces: the background layer is designed with high adhesion to the substrate and low adhesion to the security element. This localized differentiation in adhesion properties ensures that during a detachment attack, the background layer with the IR absorber remains on the substrate while the security element is removed, enabling reliable detection of tampering.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The combination of the interference layer stack and subsurface IR absorber layer forms a robust authenticity mark that can be checked by machine, ensuring that over 90% of the IR absorber remains with the document, enabling reliable detection of manipulation and preventing counterfeiting.
Implementation Method 1
an interference layer stack which gives the security element an optically variable appearance when visually observed in reflection
Implementation Method 2
a background layer containing an IR absorber, arranged at least partially beneath the security element
Data Source
Figure 1~2
Figure 3(a)~5(b)
Figure 6~7(b)
AI summary
The invention relates to a security document (10) comprising a security document substrate (12) and a security element (14) applied to the substrate. The security element (14) contains an interference layer stack (20) that gives the security element (14) an optically variable appearance when viewed visually in reflection. The interference layer stack (20) provides a characteristic IR signature as a first authentication feature, and the security document substrate (12) contains a background layer (30) on its surface, impregnated with an IR absorber (32), as a second authentication feature. This background layer is located at least partially beneath the security element (14). The first and second authentication features together form a machine-verifiable authentication mark in the infrared spectrum, the absence or incompleteness of which indicates manipulation of the security document (10).